As the use of the Internet has become popular and various kinds of information services via networks have been provided, communication networks have occupied an important status among social infrastructures. Internet accesses from ordinary homes and enterprise bases are growing, and consequently access lines connecting theses communication sites and communication offices need to be improved for high speed and large capacity.
As one of access networks to be connected to wide area networks, such as the Internet, there is the passive optical network (PON) system whereby plural subscriber terminals can share a strand of optical fiber. The PON system consists of plural ONUs (Optical Network Units) each of which accommodates a single or plural user terminals and an OLT (Optical line terminal) that is connected to these ONUs through an optical fiber network. An Optical fiber connected to the OLT is connected to branch optical fibers connected to respective ONUs via an optical splitter (optical coupler) and plural ONUs (user terminals) share an optical transmit line between the optical splitter and the OLT, which enables installation cost of optical fibers to be curtailed greatly.
The following are known as the PON system, for example: B-PON (Broadband PON) in which information is sent by a fixed-length ATM cell in the optical fiber section (PON section); G-PON (Gigabit-capable PON) that has made possible high-speed data sending of a gigabit class; and GE-PON (Gigabit Ethernet PON) suitable for information transmission by the Ethernet (registered trademark) frame that is becoming popular in LAN's and metro networks.
Both the G-PON and the GE-PON enable the sending of the variable-length frame in the PON section, and their standardization and technical examination are being carried out in ITU-T and IEEE, respectively. As an ITU-T recommendation about G-PON, there are, for example, nonpatent documents 1-3, which stipulate a GEM (G-PON Encapsulation Mode) frame standard, as a transmit frame standard whereby a general variable-length frame that is not limited by the Ethernet frame is sent to the PON section.    Nonpatent document 1: ITU-T G.984.1, “Gigabit-capable Passive Optical Networks: (GPON) General characteristics”    Nonpatent document 2: ITU-T G.984.2, “Gigabit-capable Passive Optical Networks: (GPON) Physical Media Dependent (PMD) layer specification”    Nonpatent document 3: ITU-T G.984.3, “Gigabit-capable Passive Optical Networks: (GPON) Transmission convergence layer specification”
In the PON system, a downstream frame heading from the OLT to the ONUs is branched to plural branch optical fibers by a splitter, and is broadcast to all the ONUs. Each ONU determines whether the received PON transmit frame is a frame that the local office should perform receive-process of the frame according to destination identification information shown by its header (for example, GEM header). On the other hand, upstream frames heading to the OLT from the ONU side are multiplexed to an optical fiber on the OLT side by the optical splitter. In the upstream direction communication, in order to prevent frames from overlapping on the optical fiber, the TDMA system in which each ONU is allowed to transmit a frame in a transmission time zone allocated by the OLT is adopted.
Since, as can be understood from the above-mentioned configuration, a transmit frame from the OLT is broadcast to all the ONUs in the PON system, the PON system is an access network suitable to distribute the same service information to plural user terminals by multicast. Therefore, for example, in a recently attention-receiving triple play service of broadest, telephone, and data communication, especially when broadcast industries enter network infrastructures, the PON system bears an important part as an access network whereby broadcast program information is distributed.